最新刊期
卷 27 ， 期 5 ， 2023
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On-orbit absolute radiometric calibration for optical remote sensing satellites： Progress and trends
摘要：On-orbit absolute radiometric calibration is an important prerequisite for the quantitative application of optical remote sensing satellite data. Given the complicated transfer chain of the radiometric benchmark that is difficult to realize, traditional field calibration (which takes at-ground measured target characteristics as a radiometric benchmark) may be inevitably affected by uncertainty factors, such as scale effect, atmospheric condition, and space environment perturbation. To date, the accuracy, consistency, and traceability of on-orbit radiometric calibration are still difficult to be solved. To address these problems, the “calibration benchmark satellite” can be used to carry spaceborne radiometric benchmark sensors and transfer their high-accuracy radiometric measurement values to other operational optical remote sensing satellites by synchronously observing the same at-ground targets/scenes. By precisely assessing the uncertainties introduced in various steps in the whole transfer chain, high consistency and traceability among different satellite remote sensing products can be achieved. In this study, starting with the technical requirements of on-orbit radiometric calibration for optical remote sensing satellites, the authors reviewed and described the developing processes and technical challenges related to common methods of on-orbit radiometric calibration, such as on-board, field, cross, and lunar-based calibrations. Furthermore, the most advanced calibration technology at present, namely, space-borne radiometric benchmark transfer calibration, was illustrated. In this advanced calibration scheme, radiometric benchmark sources that are traceable to the international system of units are installed in a small number of benchmark satellites. Consistent, high-accuracy, on-board radiometric benchmarks can be transferred to multi-series of operational satellites by means of cross-calibration based on synchronous observation of Earth/moon scenes. This calibration scheme is expected to extensively improve the international technical level of calibration and guarantee highly consistent and highly stable multi-source satellite data.关键词：optical remote sensing;radiometric calibration;benchmark transfer;traceability- 2099
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发布时间：2023-06-29 -
摘要：The validation of remote sensing products is a challenging and critical aspect of remote sensing research. The construction of validation infrastructure is key in validating remote sensing products. In recent years, China has successively launched the construction of the National Civil Space Infrastructure Validation Test Sites Network (Space-based Site Network) and the Gaofen National Validation Test Site Network (Gaofen Site Network), forming a network system with over 60 test sites and the ability to validate more than 40 types of remote sensing products. This paper analyzes and summarizes the validation infrastructure internationally and identifies three categories of validation infrastructure: automatic observation networks for atmospheric and ground surface parameters, observation test site networks for ecological parameters with automatic and manual measurements, and temporary experimental sites based on large-scale comprehensive experiments. The paper provides a detailed introduction to the test site selection principles, number of test sites, and product types for the Space-based Site Network and Gaofen Site Network. The differences between the two networks are also analyzed, and the preliminary results of each network are presented. The Space-based Site Network consists of 48 test sites and has the ability to validate 24 common remote sensing products in six categories. It is a business-oriented network that focuses more on long-term business measurement, using unified measurement equipment for measurement. On the contrary, the Gaofen Site Network is a research-based network with a focus on research on measurement theory and methods, as well as the development of standard specifications. It consists of 42 test sites and has the ability to validate 41 common remote sensing products in seven categories, mainly relying on the site’s own equipment for measurement. The paper proposes the idea of building a validation test site network in China based on the current construction status of the Space-based Site Network and Gaofen Site Network. It explores the technical system for building a validation test network and analyzes it from four aspects: theoretical research, standard specification formulation and optimization, validation test site network construction, and remote sensing product validation and evaluation. This paper provides a reference for the future construction of validation test site networks and the validation of remote sensing products in China. In the future, it is essential to intensify relevant theoretical research and test site network construction, and combine the development plan of validation in China to do a good job in the top-level design of the validation test site system. It is also necessary to further improve the established validation test site network system, improve the accuracy and frequency of site measurement data, accelerate the commercialization of network operation and the release of validation reports, and continuously improve the Space-based Site Network and Gaofen Site Network system to provide support for China’s Earth observation system and promote the development of quantitative remote sensing in China.关键词：remote sensing;validation;infrastructure site construction;Gaofen station network;Technical system- 920
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发布时间：2023-06-29
Review
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摘要：The TOA reflectance model based on Pseudo-Invariant Calibration Sites (PICS) can directly calculate the apparent radiance of a satellite sensor without ground measurement, thus providing an effective technical approach for high-frequency, in-orbit radiometric calibration and long-term satellite performance monitoring. However, due to the strict requirements in spatial uniformity, time stability, and atmospheric conditions, existing PICS calibration sites are all distributed abroad, and meeting the calibration requirements of Chinese land satellites, which are mostly for imaging in China, is difficult. Using the radiometric calibration idea of PICS, this study investigated the construction model of Top of Atmosphere (TOA) reflectance for inland surface stable targets with relaxed sites and atmospheric stability constraints. With the stable target of Golmud Desert in the northwest region as an example, the constraints were relaxed as follows: site area greater than 3 km × 3 km, cloud coverage less than 60%, spatial uniformity less than 3%, and temporal stability less than 10% (11 years). Then, Aqua/MODIS and European Center for Medium-range Weather Forecasting reanalysis data of 11 years were used to analyze the regularity of the observed geometry and atmospheric parameters in TOA reflectance, and the scattering angle defined in the surface/atmospheric radiation transmission model was introduced to further characterize the multiple scattering contribution between the surface and atmosphere in the TOA reflectance model. Afterward, a TOA reflectance model suitable for inland stable targets was proposed. The averaged bias and root mean square error of the model were less than 0.10% and 0.0084, respectively. In addition, the TOA reflectance directly observed by Sentinel-2A/B MSI and Landsat 8 OLI were compared with the TOA reflectance calculated by the proposed model. The average relative error between the TOA reflectance calculated by the model and the observations of Sentinel-2A/B MSI was less than 1.44%, and the standard deviation of the relative error did not exceed 1.59%. The average relative error with Landsat 8 OLI observations was less than 1.77%, and the standard deviation of the relative error did not exceed 2.11%. The validation results showed that the calculated TOA reflectance values of the proposed model had high consistency and stability with the satellite observations. Thus, the model can be applied to the on-orbit radiometric calibration and long-term radiation characteristic monitoring of other satellite payloads in the solar reflection spectrum.关键词：inland surface stable target;radiometric calibration;pseudo-invariant calibration sites;TOA reflectance;observation geometry;ECMWF- 931
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发布时间：2023-06-29 -
摘要：The objective of this paper is to develop a well-performing reference payload orbit design method for in-orbit radiometric cross-calibration. The satellite sensor is used as the target payload, and the satellite sensor with high calibration accuracy is used as the reference payload. The cross-calibration method can be carried out when the two sensors observe the same target at the same time or almost the same time. Precise atmospheric measurements are not required, but the calibration accuracy of the reference sensor is very demanding. The SGP4 orbit prediction model can be used to predict the orbits of remote sensing satellites to maximize calibration crossings. To develop a method to find the orbit of the reference payload, various factors affecting the cross-calibration frequency of remote sensing satellites need to be analyzed. The main influencing factors can then be highlighted, and the reference payload orbit optimization can be implemented with pertinence. There are two cross-calibration modes: inter-satellite cross-calibration and fixed-site cross-calibration. Based on the orbit models obtained for these modes, the effect of each of the six orbital elements on the crossover frequency can be explored by the fixed-variable method. Results indicate that the orbital altitude has the greatest influence on the crossover frequency, followed by the right ascension of ascending node and the argument of latitude, and less by the eccentricity and orbital inclination. To optimize the reference payload orbit, a method based on genetic algorithm is proposed. The optimization algorithm only needs to select the three main elements out of all six orbital elements as optimization parameters. By selecting the appropriate fitness function for the inter-satellite cross-calibration and the fixed-site cross-calibration, the overall crossover frequency and crossover uniformity of each target payload in the unit simulation cycle are significantly improved compared with that before optimization. The simulation results are helpful in carrying out the reference payload orbit optimization design for serving multi-objective payload.关键词：remote sensing satellite;orbit prediction;reference payload;cross calibration;orbit optimization- 346
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发布时间：2023-06-29 -
摘要：The HY-1 C/D satellite Coastal Zone Imager (CZI) can image a width of 950 km through the combination of multiple detector groups, which greatly improves the satellite remote sensing data acquisition capability. It is of great significance in global earth observation and ocean water color elements. However, the super-wide imaging system presents severe challenges to the full field of view relative radiometric correction processing. One of the challenges is that the ground calibration field cannot cover all the probes in the imaging field of view due to the ground coverage of the push-scan imaging being nearly 1,000 km. Another challenge is that the full field of linear array detectors and the imaging view differences can cause obvious color differences, and camera mosaic imaging can easily have more serious radiation error accumulation.The CZI is equipped with two cameras and four CCDs for combined imaging. The two CCDs inside the camera adopt collinear full reflection-full transmission optical splicing. The mirror blocks the overlapping area between the reflection and transmission regions, so energy is attenuated at the edge of the image, forming a vignette. Different focal-plane structure installation and imaging field of view, as well as different CCDs, especially the camera response difference, result in dynamic light and shade changes between images and an obvious color difference.To address this issue, an adaptive global color consistency processing method is proposed according to the characteristics of the load structure of the CZI. On the basis of single CCD image radiation correction, the color consistency processing can be carried out adaptively using the global optimization strategy. The global optimization strategy means that a single CCD image is used as a unit of all image color correction model processing, to obtain the global meaning of the optimal solution and eliminate the overall color difference between images. The proposed method first uses the on-orbit statistical method to correct the radiation of single CCD image, which improves the vignetting problem. Subsequently, high-precision geometric stitching of multiple CCD images is carried out using the large-field virtual reimaging method. On this basis, the mean and variance of the overlapping area are calculated to describe the color information of each image. Finally, according to the color consistency constraint of adjacent overlapping area before and after color correction, color parameter calculation and image balance correction of global multi CCD images are realized. In the calculation of color parameters, while eliminating the color differences between images, the color differences of the images before and after correction are ensured to be as small as possible. Doing so increases the number of necessary observation equations and solves the rank deficit problem caused by the number of overlapping pieces always being less than the number of CCD pieces.The experimental results show that root-mean-square deviation of the mean line, generalized noise method, and streaking metrics of the proposed method are better than those of the path transfer method after full field relative radiometric correction. In addition, the relative radiation quality of HY-1C/D satellite CZI is better by 3%.关键词：HY-1C/D satellite;Coastal Zone Imager(CZI);on-orbit statistical method;color consistency;adaptive equalization between ccd images;relative radiation quality- 700
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发布时间：2023-06-29 -
摘要：Networked calibration technology using the Earth surface with suitable characteristics as a reference has become a hot topic in the international calibration field. This technology can reduce the uncertainty of independent measurement by means of multiple observations, so calibration frequency and accuracy can be effectively improved. In this study, long-time series of Aqua/MODIS and Landsat8/TIRS thermal infrared data are used to systematically analyze the surface thermal radiation and atmospheric characteristics of eight targets, namely, six pseudo-invariant calibration sites recommended by the Committee on Earth Observation Satellites and the Working Group on Calibration & Validation, the Dunhuang site, and the Geermu Dazaohuo area. Meanwhile, a criterion for selecting land surface stable targets is proposed. Specifically, the criterion states that the spatial standard deviation of brightness temperature should be less than 0.3 K, and the emissivity temporal variation coefficient should be less than 2% so that land surface stable targets suitable for the networked calibration of thermal infrared payloads can be selected for improving the in-orbit calibration accuracy of thermal infrared payloads.To analyze the applicability of each target area at different spatial resolutions, spatial uniformity and temporal stability are evaluated at 1 km spatial resolution by using MODIS/MYD02_1KM and MODIS/MYD21 thermal infrared data, and the uniform and stable area is obtained. On this basis, spatial uniformity is evaluated at 100 and 500 m spatial resolutions by using Landsat8 thermal infrared data. Then, the uniform and stable regions are obtained at 100 and 500 m spatial resolutions. In addition, to analyze the scale characteristics, this study compares two brightness temperature SDs at 500 m spatial resolution with different features; one is the original 500 m, and the other one is downscaled from 100 m.Results show that the areas that meet the selection criteria at 1 km spatial resolution are Libya1_1 km, Libya4_1 km, Algeria3_1 km, Algeria5_1 km, Mauritania1_1 km, Mauritania2_1 km, Dunhuang_1 km, and Geermu_1 km. Among them, Libya4_1 km has the largest area of 75 km×75 km, and Geermu_1 km has the smallest area of 3 km×3 km. At 100 m spatial resolution, seven regions are selected further from Libya_1 km, Mauritania1_1 km, Mauritania2_1 km, and Dunhuang_1 km. In these regions, the maximum area is 30 km×30 km, and the minimum area is 5 km×5 km. In addition, the spatial uniformity of Libya1_1 km, Mauritania1_1 km, Mauritania2_1 km, and Dunhuang_1 km is not sensitive to the spatial scale, indicating that that these areas are suitable for the in-orbit calibration of thermal infrared payloads at various spatial resolutions.关键词：thermal infrared;spatial heterogeneity;Time stability;MODIS;Landsat 8;ECMWF- 834
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发布时间：2023-06-29 -
摘要：Thermal infrared radiometric calibration of satellite sensors is an important prerequisite of quantitative remote sensing. An appropriate radiometric calibration source ensures high-frequency, high-precision calibration of satellite sensors and guarantees observation stability during the on-orbit stage. Re-analysis data provide global surface and atmospheric data with a fixed resolution, and they are crucial to climate applications. The feasibility of using re-analysis data as a reference source for radiometric calibration is worthy of being studied. In this study, the ERA5 re-analysis data of the European Center for Medium-range Weather Forecasting were used. Argo buoy Sea Surface Temperature (SST) and Terra-MODIS L2 SST daily products were employed to verify the sea surface skin temperature (SSTskin) of ERA5. The MODIS-observed brightness temperature was used to verify the Top Of Atmosphere (TOA) simulation with the support of ERA5 SSTskin and atmospheric profile data. Results showed that the Mean Bias Error (MBE) between ERA5 SSTskin and Argo SST was -0.31 K, and the MBE between ERA5 SSTskin and MODIS SST was -0.38 K. The former temperature difference was more stable than the latter. The root mean square error between the simulated TOA brightness temperatures and the MODIS observations was also -0.38 K. In addition, some meteorological factors, such as wind speed, total column water vapor, and ocean wave height, were used to analyze the correlation between the SSTskin differences and TOA brightness temperature. Overall, under the conditions of medium wind speed of 6—16 m/s, total column water vapor of less than 7.0 g/cm2, and ocean wave height of 0.6—3 m, the difference between SSTskin and TOA brightness temperature was small. These findings can provide an accurate basis for the use of re-analysis data as a reference source in thermal infrared radiometric calibration.关键词：re-analysis data;sea surface skin temperature;top of atmosphere brightness temperature;radiometric calibration reference- 955
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发布时间：2023-06-29 -
摘要：Field spectroradiometers need to be traced back to a unified benchmark, namely, the international system (SI) of units, to ensure the accuracy and consistency of different field spectroradiometers. Considering that reports focusing on the accuracy of field radiometric calibration of spectroradiometers are lacking, this study investigates the field dissemination of the value of the quantity method.Uncertainty evaluation methods, such as the model of the relationship between wavelength and spectral radiance, the stray light correction model, and the temperature correction model, are provided for the transfer spectroradiometer. Then, the radiometric quantity is transferred from the laboratory measurement standard to the transfer spectroradiometer and then to the field instruments. By analyzing the uncertainty components in the whole process, a laboratory-field dissemination method of the value of the quantity chain is established, and the field spectroradiometer is traced to the SI unit uninterruptedly. In accordance with the uncertainty propagation law, the uncertainties of the transfer spectroradiometer and field spectroradiometer are obtained. Experimental results show that the uncertainty of the transfer spectroradiometer is 1.7%—2.3% (k = 1) from 380 nm to 2400 nm, and the uncertainty of the field spectroradiometer is 1.9%—2.5% (k = 1) from 380 nm to 2400 nm.The paper systematically introduces the method of field dissemination of the value of the quantity method. During the field calibration process, the uncertainty and characteristics of the standard transfer spectroradiometer are investigated. By analyzing the field calibration uncertainty in the whole process, the field spectroradiometer can be traced back to the international equivalent primary standard in the laboratory. The method is helpful for the cross verification of different types of field instruments at different sites and ensures the consistency of multiple field verification tests. It can also be used to calibrate satellite optical loads with high accuracy in the future.The field dissemination of the value of the quantity method can be divided into two partscalibration of a transfer spectroradiometer and field radiometric calibration. The influencing factors must be considered when a transfer spectroradiometer is used to calibrate the field spectroradiometer because of the differences between the laboratory and the actual site, such as relative spectral, radiation level, and temperature and humidity differences. Mathematical models are established to quantify the influence of various parameters on the spectral measurement accuracy, and a stray light correction model is built using the laser and filter method to correct the stray light caused by the relative spectral difference. The integrating sphere light source addition method is used to evaluate the nonlinearity at different radiation levels. In addition, whether spectral responsivity and detector temperature have a one-to-one correspondence is analyzed, and a temperature correction model is built according to the variation trend of spectral responsivity. After measuring and correcting the influence of the difference between the laboratory and actual site, the transfer spectroradiometer is utilized to transfer the radiometric quantity to the field spectroradiometer by using the symmetrical placement and exchange measurement method.关键词：field dissemination of the value of quantity;stray light correction model;temperature correction model;exchange measurement;Uncertainty- 608
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发布时间：2023-06-29 -
摘要：On-orbit calibration and performance monitoring of satellite remote sensing payloads call for the support of the radiometric calibration source, which has high stability, reliability, and traceability. One of the most effective ways to improve the accuracy of on-orbit radiometric calibration is to move the radiometric benchmark from the laboratory to a space-borne platform to form “calibration satellites,” such as “THUTHS,” “CLARREO,” and the Chinese “LIBRA.” Then, the simultaneous nadir overpass observations obtained from the calibration satellite and other satellites can be employed to transfer the benchmark to other satellites. However, up to now, all of the abovementioned projects are at the research and development stage. At present, no operational satellites can be used to validate the benchmark transfer chain, which is one of the core functions of future calibration satellites. Given that the high-altitude scientific balloon has the advantages of being close to TOA observations, long-term regional flight, and recyclability, it can be regarded as an optional platform for space radiometric benchmarks. This study examined the composition of the demonstration system that can operate at a near-space altitude, with the high-altitude scientific balloon as a platform and a radiometer covering the spectrum range of 400—2500 nm as the main Earth observation instrument. A flight experiment was also performed by utilizing this system in Da-Qaidam in Qinghai Province. During the flight, the position and attitude data of the balloon platform and the observed radiance data were obtained and fully recorded. These data were initially used to analyze the stability of the high-altitude scientific balloon platform and the radiometer in near-space. Results revealed that during the whole flight, the radiometer was in a stable environment and worked well. Then, a general method of satellite radiometric calibration with the balloon observation in near-space was established in consideration of balloon flight tracks that are difficult to fully control. The uncertainty of the proposed method reached 3.15%—3.35% and 4.60%—4.75% in the uniform and mountain areas, respectively. A comparison with MODIS and GF-6/WFI synchronous observations was performed to confirm the reliability of the uncertainty analysis. The satellite and balloon observations showed good agreement with each other. The successful flight experiment revealed the feasibility of using high-altitude scientific balloons as a space radiometric benchmark-carrying platform. It can also serve as a reference for the further development of near-space-borne radiometric benchmark transfer calibration systems.关键词：remote sensing;Near-space;high-altitude science balloon;radiometric calibration;field experiment- 716
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发布时间：2023-06-29
On-orbit Calibration Techniques and Methods
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摘要：The aim of this study is to obtain a reliable, high-precision radiometric calibration coefficient for the GF-7 satellite sensor. In this study, we describe the methods and results of a reflectance-based vicarious calibration campaign that was conducted in September 2020 at RadCalNet Baotou Site located in Inner Mongolia, China. A series of gray-scale permanent artificial targets and infrastructure have been built in the Baotou site to provide effective support for the GF-7 satellite sensor’s on-orbit calibration. The reflectance-based vicarious calibration approach relies on the synchronous measurement of surface spectral reflectance and atmospheric parameters. In this campaign, synchronous measurements of surface and atmospheric conditions (including aerosol optical depth, amount of water vapor, and aerosol inversion products) at the Baotou site at the time of the GF-7 satellite’s overhead pass are obtained with an SVC HR-1024i spectroradiometer and a Cimel CE318 sun photometer. The bidirectional reflectance distribution function (BRDF) of the surface is measured with a multi-angle instrument with an SVC spectroradiometer. The synchronous measurement of surface spectral reflectance is modified by the surface BRDF model, and the atmospheric parameters are coupled with the exoatmospheric solar irradiance spectrum. The relative spectral response of the sensor is adopted as an input for a radiative transfer model to compute the at-sensor spectral radiance. The relationship between the at-sensor radiance and the digital number (DN) recorded in the GF-7 satellite image is identified. Results show a good linear relationship between DN and the apparent radiances of each band. The uncertainty of the GF-7 sensor radiometric calibration in the RadCalNet Baotou site is 4.33%, which is less than 5%. A detailed discussion of the validation analysis of the GF-7 radiometric calibration coefficient is performed using the products from the automated RadCalNet Baotou Sand site. This study can provide a site reference for the radiometric validation of Chinese satellites. In consideration of the results shown in this paper, the data from the RadCalNet Baotou site can be used as a reference to evaluate the radiometric performance of Chinese high-resolution satellites.关键词：remote sensing;radiometric calibration;TOA radiance;reflectance-based approach;the Baotou site;GF-7 satellite- 1088
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发布时间：2023-06-29 -
摘要：GF-4 is the first high-resolution geostationary orbit optical remote sensing satellite in China, and it is equipped with a staring camera that covers the visible-to-mid-infrared spectrum. Providing in-orbit radiometric calibration coefficients frequently is difficult due to the lack of on-board calibration devices. To effectively monitor the radiometric performance of GF-4/PMS and provide a solid basis for quantitative applications, the cross-calibration method was used in this study to derive the calibration coefficients over a selected calibration site located in the north of Badan Jaran Desert. The cross-calibration site was identified by searching the uniform area in a high-resolution image (i.e., Landsat 8 /OLI) to minimize the uniform impact on the cross-calibration results. To reduce the effects of the imaging angles of the sensor and atmospheric changes, the restraint was set to the imaging angle difference between MODIS and GF-4 (less than 20°), the imaging time difference (less than 2 h), the cloud, and image quality. In total, 13 image pairs were available from 2016 to 2018 and were used to calculate the cross-calibration coefficients after compensating for the spectral matching factors, which were simulated by MODTRAN. Results showed that (1) the cross-calibration coefficients calculated under the Lambertian assumption were highly consistent with those calculated under the Ross-Li BRDF assumption. The uncertainty was less than 7.4%, and the relative difference ranged between 0.8% and 4% when BRDF was neglected. (2) The radiometric performance of GF-4/PMS decreased slowly from 2016 to 2018, with 1% decrement per year. Thus, the proposed method can effectively improve the radiometric calibration frequency of GF-4/PMS with an acceptable accuracy and can be used for radiometric performance monitoring over the whole life cycle of the sensor.关键词：remote sensing;cross calibration;GF-4/PMS;MODIS;uncertainty analysis- 785
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发布时间：2023-06-29 -
摘要：Soil Moisture Active Passive (SMAP) Satellite and Soil Moisture and Ocean Salinity (SMOS) Satellite are passive microwave radiometers that operate at the L band. They can be used to estimate sea ice parameters, such as sea ice thickness, sea ice concentration, and snow depth on sea ice, so they play an important role in monitoring sea ice parameters and their changes in polar regions. A comprehensive comparison of SMAP and SMOS brightness temperatures (TBs) is necessary because it can help identify possible deficiencies in TB products and construct a highly consistent and reliable TB dataset for sea ice monitoring.In this study, the sea region north of 55°N in the Arctic was selected as the study area. Top Of Atmosphere (TOA) TB observations derived from SMAP L1B and SMOS L1C products were compared for the period of October 2015 to October 2020. Given that SMAP measures at a fixed incidence angle of 40°, SMOS observations at incidence angles between 37.5° and 42.5° were averaged and compared with SMAP TBs. The discrepancy between SMAP and SMOS TBs was evaluated by computing the Pearson correlation coefficient (r), bias (SMOS minus SMAP), and root mean squared deviation (RMSD). The dependence of the discrepancy parameters on polarization, sea ice type, and season was also investigated. Given the higher radiometric accuracy of SMAP compared with that of SMOS, the SMAP observations were used as the reference data, and the SMOS TBs were calibrated using a linear regression method, with the slope and intercept values provided for each month. The RMSD values between the calibrated SMOS and SMAP were evaluated at both polarizations for different sea ice types.Results indicated that the brightness temperatures of SMAP were generally lower than those of SMOS, with bias being 2.0—3.0 K and 3.0—4.5 K for H and V polarizations, respectively, and RMSD being 4.5—6.0 K for H and 5.0—6.0 K for V. During wintertime (October to April), multi-year ice (MYI) had the lowest bias and RMSD values, followed by first-year ice (FYI). Open water (OW) had the highest bias and RMSD. During summertime (May to September), the bias and RMSD values for sea ice were similar to those for OW. The slope and intercept values for calibrating the SMOS TBs showed strong seasonal variation. However, their inter-annual variabilities for each month were small, so averaged calibration coefficients could be achieved for each month. The obtained slope and intercept values were used to calibrate the SMOS TBs, and the results showed that the calibrated SMOS agreed well with SMAP, with the overall RMSD being 4.4 K and 3.8 K for H and V polarization, respectively. However, the different sea ice types had different RMSD values. Compared with MYI that had the lowest RMSD values, i.e., 2.4 K for H and 1.9 K for V, FYI had much higher RMSD values of 4.5 K and 3.9 K for H and V, respectively. Moreover, the calibration accuracy at the end of the sea ice growth stage (January to April) was higher than that at the beginning of the sea ice growth stage (October to December).This study helps understand the discrepancy between SMAP and SMOS TB observations. The obtained calibration coefficients can be used to calibrate SMOS and contribute to the construction of a long-time-series, L-band, consistent-brightness temperature dataset for Arctic sea ice monitoring.关键词：remote sening;SMAP;SMOS;brightness temperature;consistency;inter-calibration- 806
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发布时间：2023-06-29 -
摘要：Spaceborne laser altimeter plays an important role in stereo mapping services and in the measurement of ice caps, vegetation height, sea surface, and so on. Spaceborne laser altimetry, an Earth observation technology, has been widely used in America for about 20 years. However, China has used only experimental spaceborne laser altimetry for Earth observation. GF-7 is the first Earth observation laser altimeter of China, and it is equipped with full-waveform lidar. The data of spaceborne laser altimeters can be regarded as control points to realize the requirement of 1∶10000 stereoscopic mapping under the condition of few control points. In this study, a spaceborne laser altimetry geo-location model and an on-orbit geometric calibration and verification method are proposed to meet the high processing precision requirement of the first full-waveform spaceborne laser altimeter of China and realize the localization of geometric positioning and data processing for full-waveform spaceborne laser altimetry.First, a rigorous geometric model for the GF-7 spaceborne laser altimeter is built. Second, the peak value extraction method of moving the center of gravity and waveform decomposition are used. Third, the calibration method of the GF-7 spaceborne laser beam is established based on terrain matching and infrared detection. Lastly, comprehensive validation methods, including SRTM (Shuttle Radar Topography Mission)elevation verification, lake elevation verification, and calibration site elevation verification, are put forward to verify the positioning accuracy of the spaceborne laser altimeter comprehensively.The suitable data of the GF-7 spaceborne laser altimeter in Neimenggu Province and Qinghai Lake are selected, and IR detectors are placed in a field in Neimenggu Province. A series of on-orbit geometric calibration and verification experiments are conducted. The experiments show that the corresponding pointing angle error of Laser 1 is about 0.15", and that of Laser 2 is about 0.38". After correcting the pointing angle error, the height measurement error of the laser altimeter is better than 0.15 m in the flat area. GF-7, the first spaceborne laser altimeter for Earth observation in China, has good performance. It can realize high-precision positioning and data processing for full-waveform spaceborne laser altimetry.The results also show that the method in this study has high processing accuracy and realizes the first domestic processing of full-waveform spaceborne laser altimetry data. The processed data can satisfy the requirements of scientific research and subsequent large-scale application. Moreover, the geo-location model for GF-7 and the calibration verification method established in this study can be used for domestic spaceborne laser altimetry in the future.关键词：GF-7 satellite;Laser altimeter;Rigorous geometric model;Geometric calibration;accuracy verification- 958
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发布时间：2023-06-29 -
摘要：Crop evapotranspiration (ET) with high precision is of great significance for the accurate quantification of water balance and the study of water deficit in the field-scale, and it has practical value for the precision irrigation of farmland and the improvement of agricultural water use efficiency. It is essential to validate ET before a remotely sensed ET product being used. This study evaluated crop ETs from two remotely sensed products (MOD16 and PML-V2) with 500 m spatial resolution and 8-day temporal resolution by using Eddy-Covariance (EC) flux observations from 28 flux tower sites cover with crop globally. The results showed that, compared with the observed ET, the Root Mean Square Error (RMSE) and bias of the PML-V2 ET products varied from 3.3 to 22.4 mm/8 d and from 15.98 to 13.27 mm/8 d, respectively, while the RMSE and bias of the MOD16 ET product varied from 3.81 to 21.47 mm/8 d and from -16.42 to 15.05 mm/8 d, respectively. On the whole, the overall accuracies of these two products were similar, the MOD16 product underestimated the 8-day ET with a bias of -2.31 mm/8 d, a R2 of 0.452 and a RMSE of 8.82 mm/8 d, while the PML-V2 product slightly overestimated the 8-day ET with a bias of 0.51 mm/8 d, a R2 of 0.455 and a RMSE of 8.81 mm/8 d. The PML-V2 product performed better at 18 tower sites (almost 64%), but the MOD16 product performed better than the PML-V2 products at some sites in the depiction of details on time-series change (such as the season of reaching the peak during the year, the decreasing and increasing trend in the middle of year). The results showed that the PML-V2 product failed to capture the gradual decrease then increase ET trend in the middle of year which caused by the rotation of winter wheat and summer maize, while the MOD16 product successfully captured the hitting of the two peaks in ET time-series during the two growth seasons of winter wheat and summer maize (such as the Luancheng and Yucheng sites). However, the MOD16 product still underestimated the 8-day ET of winter wheat to a certain degree. Moreover, the results showed that both the MOD16 product and the PML-V2 product seriously underestimated the 8-day ET of paddy with a RMSE of 21.47—22.4 mm/8 d and a bias of -16.42—-15.98 mm/8 d (such as the US-Twt site). This study could provide reference for the development and validation of ET models for cropland. In the future, more detailed evaluation of land surface heterogeneity needs to be carried out and more products should be taken into consideration. Further detailed evaluation of ET in different crop types is also required.关键词：remote sensing;MOD16 product;PML-V2 product;crop evapotranspiration;Validation- 760
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发布时间：2023-06-29 -
摘要：The Clumping Index (CI) represents the degree of leaf aggregation in a canopy. It is an important vegetation structural parameter that affects radiative transport and is crucial to understanding radiative transport in canopies. The apparent clumping index (ACI) was proposed to deepen the understanding of the clumping effect and expand the field indirect measurement of CI. Studies have shown that ACI is closely related to the angular sampling scale. Even ACI measured by the same instrument changes considerably with the sampling scale, which presents a new challenge to the verification of CI products. The combination of digital hemispherical photography (DHP) and logarithmic average methods is widely used in field ACI estimation, and one of the key steps of this combination is the determination of the angular sampling resolution. At present, the influence of angular sampling resolution on ACI estimation and its variation with different vegetation types need to be further explored.When using the logarithmic average method based on DHP measurement data, the angular sampling resolution is jointly determined by two factors: view zenith angle sampling scale (∆θ) and view azimuth angle sampling scale (∆ϕ). On the basis of DHP images, this study divided view zenith and view azimuth angles by using a series of progressively increasing angular sampling scales. A total of 30 sampling methods and 17 levels of angular sampling resolutions were obtained and applied to forest, shrub, grass, and crops. The variation in ACI under different vegetation types was investigated with the decrease in the resolution of the 17 levels of sampling. At the same time, the relationship among the angular sampling scale effect of ACI, the spatial distribution of vegetation, and the growth stage of crops was discussed.Results showed that ACI varied from 0.5 to 0.8 in the four vegetation types with the decrease in the 17-level angular sampling resolution and exhibited an average increase of 26%, 29%, 14%, and 35%. At the different growth stages of soybean crops, ACI increased (up to 60%) with the decrease in the 17-level angular sampling resolution. In general, ACI was directionally dependent on the change in the angular sampling scale, and this characteristic was affected by the spatial distribution of the observed objects. When the observed objects were randomly distributed (forest, shrub, and grass), ACI generally showed a linear growth trend with the increase in VZA and VAA sampling scales; when the observed object had a regular distribution (soybean crop), ACI was still affected by VZA and VAA sampling scales, and the influence of the VZA sampling scale on ACI was highly significant.The results of this study showed that ACI increased with the increase in the angular sampling scale, that is, with the increase in the angular sampling scale, vegetation aggregation weakened and gradually exhibited a random distribution. This study further explored the directional dependence of ACI on the change in the angular sampling scale for different vegetation types. How to select the optimal angular sampling scale for different vegetation types is expected be the focus of future research. A detailed analysis of ACI field measurement and an in-depth discussion of the possible uncertainties in the measurement process are prerequisites for performing an authenticity inspection of CI products. They are also essential for improving the accuracy of CI estimation and CI product verification.关键词：Clumping Index (CI);Apparent Clumping Index (ACI);Digital Hemispherical Photography (DHP);angular sampling scale;scale-effect;directionality- 814
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发布时间：2023-06-29
Satellite Calibration Instances and Groud-based Validation
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